The demand for light olefins, such as propylene and butylenes, and particularly propylene, is increasing faster than present plant capacity. A major source of propylene is from fluid cat cracking (FCC) processes. Fluid cat cracking is an established and widely used process in the petroleum refining industry, primarily for converting petroleum oils of relatively high boiling point, to more valuable lower boiling products, including gasoline and middle distillates such as kerosene, jet fuel and heating oil. In an FCC process, a preheated feed is brought into contact with a hot cracking catalyst, which is in the form of a fluidized, fine powder, in a reaction zone which comprises a riser. Cracking reactions are extremely fast and take place within three to five seconds. The heavy feed is cracked to lower boiling components, including fuels, light olefins, and coke. The coke and cracked products which are not volatile at the cracking conditions, deposit on the catalyst. The riser exits into a separator-stripper vessel, in which the coked catalyst is separated from the volatile reaction products and stripped with steam. The stream strips off the strippable non-volatiles and the stripped catalyst is passed into a regenerator in which the coke and any remaining hydrocarbonaceous material are burned off with air, or a mixture of air and oxygen, to form a regenerated catalyst. This regeneration heats the catalyst for the cracking reactions and the hot, regenerated catalyst is returned to the riser reaction zone. The process is continuous. Thus, a typical FCC cracking unit includes (i) a riser (ii) a separation-stripping vessel and (iii) a regeneration vessel. Some FCC units include two risers, so as to have two reaction zones for catalytically cracking the FCC feed, in association with a single separation-stripping vessel and a single catalyst regeneration vessel. Feeds commonly used with FCC processes are gas oils which are high boiling, non-residual oils and include straight run (atmospheric) gas oil, vacuum gas oil, and coker gas oils. Typical FCC cracking catalysts are based on zeolites, especially the large pore synthetic faujasites, such as zeolites X and Y. The olefins yield from the cracking reaction is limited by the process and cracking catalyst. U.S. Pat. No. 3,928,172 discloses an FCC process with increased light olefin production. The process includes a cracking catalyst containing faujasite and ZSM-5 zeolite components, a first riser for cracking the FCC feed and a second riser for cracking naphtha produced in the first riser. Cracking the naphtha in the second riser produces more olefins and improves the naphtha octane. In all the embodiments, the second riser is associated with a separate or outboard vessel, and not with the separation-stripping vessel used with the first riser. While it is possible to build a new FCC unit with additional risers and vessels for increased light olefins production, it is extremely costly to add additional vessels to an existing FCC unit. Therefore, it would be beneficial to be able to increase the light olefins yield from an existing FCC unit, without having to add additional vessels to the unit.